Math Problem Statement

Determine if the given series converge or diverge. If the series converges, find the sum: 8 + 6 + 9/2 + 27/8 + ...

Solution

The given series is:

8+6+92+278+8 + 6 + \frac{9}{2} + \frac{27}{8} + \dots

We need to determine if the series converges or diverges, and if it converges, find its sum.

Step 1: Identify the type of series

This appears to be a geometric series. A geometric series has the form:

a+ar+ar2+ar3+a + ar + ar^2 + ar^3 + \dots

where aa is the first term, and rr is the common ratio between consecutive terms. Let's check if there's a common ratio between the terms.

  • First term a=8a = 8.
  • Second term 6=8×r6 = 8 \times r, so r=68=34r = \frac{6}{8} = \frac{3}{4}.
  • Third term 92=6×r\frac{9}{2} = 6 \times r, so r=926=912=34r = \frac{\frac{9}{2}}{6} = \frac{9}{12} = \frac{3}{4}.
  • Fourth term 278=92×r\frac{27}{8} = \frac{9}{2} \times r, so r=27892=278×29=34r = \frac{\frac{27}{8}}{\frac{9}{2}} = \frac{27}{8} \times \frac{2}{9} = \frac{3}{4}.

Thus, the common ratio r=34r = \frac{3}{4}, confirming that this is a geometric series.

Step 2: Test for convergence

A geometric series converges if the absolute value of the common ratio r<1|r| < 1. In this case:

34=34<1\left| \frac{3}{4} \right| = \frac{3}{4} < 1

So, the series converges.

Step 3: Find the sum of the series

The sum SS of an infinite geometric series is given by the formula:

S=a1rS = \frac{a}{1 - r}

where aa is the first term and rr is the common ratio. Substituting the values:

S=8134=814=8×4=32S = \frac{8}{1 - \frac{3}{4}} = \frac{8}{\frac{1}{4}} = 8 \times 4 = 32

Thus, the sum of the series is 32.


Would you like more details or have any questions? Here are some related questions to explore:

  1. How do you test for convergence in other types of series?
  2. What happens to a geometric series when the ratio rr is greater than or equal to 1?
  3. Can a geometric series have negative terms and still converge?
  4. How is the sum formula for a finite geometric series derived?
  5. What are some real-world applications of geometric series?

Tip: In a geometric series, when r<1|r| < 1, each successive term becomes smaller and smaller, ensuring the series converges.

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Math Problem Analysis

Mathematical Concepts

Geometric Series
Series Convergence

Formulas

Sum of Infinite Geometric Series S = a / (1 - r)

Theorems

Geometric Series Convergence Theorem

Suitable Grade Level

Grades 11-12